lockd: Use xdrgen XDR functions for the NLMv4 NM_LOCK procedure

lockd: Use xdrgen XDR functions for the NLMv4 NM_LOCK procedure

Now that nlm4svc_do_lock() has been introduced to handle both
monitored and non-monitored lock requests, the NLMv4 NM_LOCK
procedure can be converted to use xdrgen-generated XDR
functions. This conversion allows the removal of
__nlm4svc_proc_lock(), a helper function that was previously
shared between the LOCK and NM_LOCK procedures.

Replace the NLMPROC4_NM_LOCK entry in the nlm_procedures4
array with an entry that uses xdrgen-built XDR decoders and
encoders. The procedure handler is updated to call
nlm4svc_do_lock() directly and access arguments through the
argp->xdrgen hierarchy.

The .pc_argzero field is set to zero because xdrgen decoders
fully initialize all fields in argp->xdrgen, making the early
defensive memset unnecessary. The remaining argp fields that
fall outside the xdrgen structures are cleared explicitly as
needed.

Reviewed-by: Jeff Layton <jlayton@kernel.org>
Signed-off-by: Chuck Lever <chuck.lever@oracle.com>